In recent years, downsized mobile devices have spread worldwide, and simultaneously, the demand for downsized large-capacity nonvolatile memory has rapidly enlarged with the drastic progress of high-speed telecommunications networks. Therein, NAND flash memory and downsized HDDs (hard disk drives) in particular have achieved rapid advancements of recording density to form a large market.
Under such circumstances, several ideas for new memory aiming to drastically exceed the limits of recording density have been proposed. For example, PRAM (phase-change random access memory) employs the principle of using a recording material capable of having the two states of an amorphous state (ON) and a crystalline state (OFF) and recording data by having these two states correspond to the binary data of “0” and “1.”
To write/erase, for example, the amorphous state is made by applying a high-power pulse to the recording material; and the crystalline state is made by applying a low-power pulse to the recording material.
The reading is performed by providing a reading current small enough not to cause writing/erasing of the recording material and by measuring the electrical resistance of the recording material. The resistance value of the recording material of the amorphous state is greater than the resistance value of the recording material of the crystalline state, and the ratio thereof is about 103.
The greatest merit of PRAM is in the point that the operation is possible even when the device size is reduced to about 10 nm; and in such a case, because a recording density of about 10 Tbpsi (terra bit per square inch) can be realized, it is considered to be one candidate to achieve high recording density (for example, refer to Patent Citation 1).
Also, although different from PRAM, a new memory having an extremely similar operation principle has been reported (for example, refer to Patent Citation 2).
According to this report, nickel oxide is a typical example of the recording material on which data is recorded, and a high-power pulse and a low-power pulse are used for the writing/erasing similarly to PRAM. In such a case, an advantage is reported that the power consumption during the writing/erasing is reduced in comparison to PRAM.
Although the operation mechanism of this new memory has not been elucidated to date, the reproducibility has been confirmed, and it is considered to be one candidate to achieve high recording density. Several groups are attempting to elucidate the operation mechanism.
In addition thereto, MEMS memory using MEMS (micro electro mechanical systems) technology has been proposed (for example, refer to Non Patent Citation 1).
In particular, a MEMS memory referred to as Millipede has a structure in which multiple cantilevers in an array configuration oppose a recording medium coated with an organic substance, and probes of the tips of the cantilevers contact the recording medium with moderate pressure.
The writing is performed by selectively controlling the temperature of heaters added to the probes. In other words, when the heater temperature increases, the recording medium softens, the probe sinks into the recording medium, and a depression is made in the recording medium.
The reading is performed by scanning with the probes over the surface of the recording medium while providing a current to the probes such as not to cause the recording medium to soften. Because the probe temperature decreases and the resistance value of the heater increases when the probe drops into a depression of the recording medium, data can be sensed by reading the change of the resistance value.
The greatest merit of MEMS memory such as the Millipede is in the point that the recording density can be drastically improved because it is not necessary to provide an interconnect to each recording unit that records the bit data. Currently, a recording density of about 1 Tbpsi has already been achieved (for example, refer to Non Patent Citation 2).
Also, since the publication of the Millipede, recently, it is being attempted to achieve large improvements of power consumption, recording density, operation speed, and the like by combining MEMS technology with new recording principles.
For example, a method has been proposed to perform the recording of data by providing a ferroelectric layer in the recording medium and causing a dielectric polarization in the ferroelectric layer by applying a voltage to the recording medium. According to this method, there are theoretical predictions that the spacing between recording units recording the bit data (the minimum unit of recording) may approach the unit cell level of the crystal.
If the minimum unit of recording is one unit cell of the crystal of the ferroelectric layer, the recording density becomes the huge value of about 4 Pbpsi (peta bit per square inch).
However, MEMS memory recording by such a ferroelectric, while being a conventionally known principle, still has yet to be realized.
The greatest reason therefor is that the electric field emitted by the recording medium to the outside is undesirably obstructed by ions in the air. That is, because the electric field from the recording medium cannot be sensed, the reading cannot be performed.
Another reason is that in the case where lattice defects exist in the crystal, charge due to lattice defects moves into the recording unit and undesirably obstructs the charge.
The problem of the electric field obstruction by ions in the air of the former has recently been solved by a proposal of a reading method using an SNDM (scanning nonlinear dielectric microscope); and this new memory has progressed substantially toward practical use (for example, refer to Non Patent Citation 3)    Patent Citation 1: JP-A 2005-252068 (Kokai)    Patent Citation 2: JP-A 2004-234707 (Kokai)    Non Patent Citation 1: P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz and G. K. Binnig, IEEE Trans, Nanotechnology 1, 39 (2002)    Non Patent Citation 2: P. Vettiger, T. Albrecht, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, D. Jubin, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz, D. Wiesmann and G. K. Binnig, P. Bachtold, G. Cherubini, C. Hagleitner, T. Loeliger, A. Pantazi, H. Pozidis and E. Eleftheriou, in Technical Digest, IEDM03 pp. 763-766    Non Patent Citation 3: A. Onoue, S. Hashimoto, Y. Chu, Mat. Sci. Eng. B120, 130(2005)